1. Field of the Invention
The present invention generally relates to a method and an apparatus for reconstructing a SPECT (single photon emission computed tomography) image, i.e., a three-dimensional distribution image of a radioisotope which has been injected into a biological body under medical examination, by detecting .gamma.(gamma)-rays emitted from the injected radioisotope. More specifically, the present invention is directed to a SPECT image reconstructing method capable of reconstructing a SPECT image with high spatial resolution by way of the filtered back projection method, and also to such a SPECT image reconstructing apparatus.
2. Description of the Prior Art
SPECT systems include one or more two-dimensional detectors which rotate around a patient and detect radiation emitted from a radioisotope injected into the patient as projection data and perform reconstruction of multiple-slice images of radioisotope distribution in the patient through the convolution and back-projection of the projection data. In such a SPECT imaging system, a fan-beam collimator is used to increase the sensitivity over that of a parallel-hole collimator without sacrificing resolution.
There are several image reconstruction methods known for SPECT images which were originally developed for .gamma.-ray CT scanners. The publication "CONVOLUTION RECONSTRUCTION TECHNIQUES FOR DIVERGENT BEAMS" by G. T. Herman et al., Comput. Biol. Med. Vol. 6; 1976, pages 259-271, describes the technique of fan-beam reconstruction for .gamma.-ray CT scanners. The fan-beam reconstruction method includes convolution and back-projection of projection data without reordering or rearranging projection data detected in fan-beam shape into parallel-beam geometry. The fan-beam reconstruction method has an advantage over the parallel-beam reconstruction method in that projection data detected in fan-beam shape may be rearranged into parallel-beam geometry since no error due to such rearrangement occurs in the fan-beam reconstruction method.
To further to improve a SPECT image resolution, a weighted back-projection method has been attempted. In an article "TRIANGULAR SPECT SYSTEM FOR 3-D TOTAL ORGAN VOLUME IMAGING: DESIGN CONCEPT AND PRELIMINARY IMAGING RESULTS" by C. B. Lim et al., IEEE Transactions on Nuclear Science Vol. NS-32. No.1, February 1985, pages 741-747, a distant-dependent weighting function favoring closer distance to a detector is employed in both parallel-beam and fan-beam reconstructions. However, the article has a negative opinion about such a weighted back-projection as mentioned on page 743, left column, lines 9-4 from the bottom of the second paragraph: "One drawback of the weighted back-projection is the image noise increase due to uneven weighting of the opposing projection image noises within uneven weighting of the opposing projection image noises within SPECT image plane. Therefore in an extremely photon-deficient situation, the weighted back-projection may not be preferable."